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The Wind Trubine
The Wind Turbine Generator (WTG) is the machine capable of turning wind into electricity, there are thousands of
WTG's of different sizes deployed around the globe and most of them share a particular set of characteristics.
This paper illustrates the main components of a WTG, and how they work in order to convert an air ow into usable
electricity, in addition of a brief introduction of the main principles that make this technology possible, considering
some of the main constrains of the technology.
Designed and created by César Sierra
csierra@simplerenewables.com
First Published in 14/04/2021
Version 1.1
Last updated in 21/04/2021
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Wind
Rotor
The wind that acts on the blade
airfoils is composed from the natural
ow of air and the ow of air
produced by the rotation of the blades
The further away from the hub the
greater the rotational component
of the wind
Wind turbine blades are
composed of multiple airfoils
Hub
Nacelle
The nacelle and the blades continuously
align themselves to maximize the ow of air
through the blades
Perpendicular to the
ow of air is the lift
Blade
Parallel to the ow
of air is the drag
An airfoil is solid geometrical shape
that generate aerodynamic forces
Tower
Modern onshore turbines have hub heights
around 120m and continue to rise each year
Boeing 747-8
Boeing 747-8 has
a length of 76.4m
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Power output and efciency
An oversimplication of how WTG generate energy, is partially given by the power formula which is derived from
the kinetic energy formula. However, the turbine components efciency must be considered to obtain the nal power
output. The power formula is dependent of 3 main things, rotor diameter, air density and wind speed.
1 m v2
E = __
2
Wind turbine blades extract energy
form wind particles crossing the swept
area, the particles that hit the blades
transmit energy by reducing their
speed, since their mass is constant.
v
m
m
E
P = ___
t
_1 mv2
2
P = ___
t
d
t
=v
Wind speed affects the power output more
than any other factor, the derivation of the
mas ow and kinetic energy result
in a cubic wind speed.
V = A1
A
The swept area is given by the length of
the blades and it is the main mechanical
improvement that increase the power
of a turbine.
1meter
1 m v2
E = __
2
m = Vρ
m = A1ρ
m = Mass ow = V ρ v
__
t
m = A1ρ v
__
t
2
1
P = _ A1ρ v v
2
3
1
P = _ A ρv
2
altitude
temperature
atmospheric
pressure
ρair
humidity
The air density is dependent of
multiple factors, the altitude
and temperature are the
most inuential.
1
__
6
2
__
6
3
__
6
vinitial = 1
4
__
6
Initial speed
5
__
6
vnal = 1/3
Final speed
6
__
6
3V
__
3
Theoretical
energy
extracted
Not all wind is affected by the
WTG and not all energy can
be extracted from the wind
2
2
2V x
__
88% = 59.26% Betz limit
3
= (1) - (1/3) = 88.88%
2
2
= vinitial - vnal
The most optimal WTG would reduce 2/3 of the
wind speed, from 2/3 of the air volume passing
through the swept area, this is the theoretical limit
of energy that a turbine can extract form the wind.
Modern turbines efciency
Generator
Other components
WTGefciency
Gearbox
Blade design
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30% - 50%
3
1
_
WTGefciency
A
ρv
P=
2
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Blade design
Faster, longer or thicker, Ideal rotors should be allowed to spin faster and with long blades that cut through air with
minimum drag, nevertheless these traits come with their constrains. Modern turbines are optimized to generate as
much torque possible within the possibilities of their mechanical and environmental constrains.
Depending on the wind
speeds, rotational speeds
or angle of attack, airfoils
work differently, ideally
they generate the maximum
lift possible for the lowest
drag possible
Lift vs Drag
Slim airfoils are optimal for high-speed
rotations generating low drags
Blade Length
The sweet spot lies in blades
that combine multiple airfoils
according to their wind
speeds
Thicker airfoils generate higher lifts
even in low speeds although they
tend to have large drags
The main constrain of longer blades come
with the materials stiffness vs cost, long elastic
blades could hit the WTG tower in high winds
Thicker airfoils require additional structure to maintain their form
while slimer airfoils may not have enough space for structure
and require expensive materials to compensate
Longer blades generate more torque as the
lift momentum increases with the length
Rotational speed
High rotational speeds are ideal
as blades are able to extract more
energy from the wind and even
generate less turbulence, however
this comes with an increase on
mechanical stress and heavy noise
Fast rotating blades disrupt
air ow less but can extract
more energy from it
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Naturally turbines want to
rotate faster, similar to a
pinwheel, the faster you
blow the faster they rotate
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The acting forces
Due to their geometry and orientation the lift and drag forces decompose into different components regarding the
WTG's orientation, in the end two forces result, torque and thrust, the rst one rotates the generator, the second one
is “absorbed” by the structural components of the turbine.
3 different planes act over the airfoil, the direction of the wind,
the direction of the airfoil and the direction of the rotation
The lift and drag components on the rotation plane are
the ones that affect the energy output of the WTG
The difference between wind direction and airfoil direction is called
angle of attack, this is controlled by the pitch of the blades
Wind ow
The lift and drag component on the rotation
plane will result on 3 forces, a lift component,
drag component and a thrust component,
the rst two will subtract resulting on a nal
acting lift that contributes to the torque
Lift Theory
The same way as an airplane air molecules
passing through a blade create different
pressure zones that generate the lift
Rotation air ow
LIFT
Natural air ow
+
=
DRAG
Both lift and drag component
will add on the thrust plane,
the thrust is the force trying
to tip over the WTG
Rotation plane
Wind direction
Airfoil direction
L
The further away from the hub a lifting force acts, the greater
the torque force it will generate, on the same way the thrust
forces will act on the turbine structure
L
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Yaw and Pitch control
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Yaw and pitch control are essential for an optimal energy production, any misalignment in the blades angle of
attack (pitch) or in the turbines direction towards the wind ow (yaw) will result in a diminish of torque in addition to
unwanted stress in the structural components. WTG's have systems that continuously change yaw and pitch to
optimize production
Near the blade tip, the air ow
produced by the rotation is higher
than the natural air ow
The correct angle of attack will
maintain the best lift vs drag
relation, maximizing torque
and minimizing thrust
Blades are twisted along their length,
this way the entire blade is oriented
towards its ideal angle
Near the hub rotational air ow is minimum,
but the natural ow maintains the same
speed through the blade, the perceived
wind ow direction changes
Pitch
If the blade is under pitched it will
generate a low amount of Torque
and Thrust, this can be useful to slow
down the blades or to stand in
idle position
This is a common distribution
of Torque and Thrust forces
along the blade
Yaw
If the WTG is not aligned
correctly towards the incoming
wind (yaw), the blades will
perceive wind differently in
each stage of the rotation,
this will diminish their
production and produce
unwanted structural loads
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When the blades are over
pitched they will generate an
immense amount of force,
however the drag component
will diminish the additional lift
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Main Components
What's inside the box? Most modern WTG's share similar characteristics, the size, location and layout varies among
manufacturers but most modern turbines look alike, most components are located inside the nacelle but some can also
be found at the bottom of the tower or even outside the turbine
Control system
Control cables
Gearbox
Low speed rotor
Power
electronics
Holding disk
brake
Generator
High speed
rotor
Electric cables
Condition monitoring
sensors
Ventilation system
Rotor
Bearings
Cooling system
Aviation lights
Access hatch
Main inner
structure
Fire suppression
system
Nacelle
MET
sensors
Hub
Pitch control
motors
Outer vents
Yaw control
motors
Access shaft
Service elevator
Tower
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Power Curve
The mind and soul of the WTG lies in its generating system, the controller unit which acts as the brain, uses a network
of sensors to monitor and orchestrate the functionality of the main components, this allows WTG to convert variable
wind speed into reliable and usable high voltage alternating electric current. The nal power output of the WTG is
plotted in a graph called the power curve, this graph demonstrates de capabilities of the system for a different
range of conditions, the power curve is essential for the development and commissioning of wind farms.
Gearbox
Controller & sensors
Most WTG use a single stage
planetary gearbox, that
transform the low speed rotation
of the blades into the high speed
rotation needed by the generator
A continuous monitoring system is in the core of the controller, measuring
rotation speeds, wind speeds, vibrations, temperatures, power outputs, etc,
processing al the data inputs and adjusting the WTG as needed
Variable
alternating current
Direct current
Synchronized
alternating current
Doble fed asynchronous generator
Power Electronics
Wind turbine generators differ form conventional generators as
they are asynchronous, since the rotational speed that feeds
them is not constant, power electronics are used to compensate
by feeding one of the windings (Stator / rotor) with
alternative current, that constantly variates in phase and
frequency, these variations are calculated to compensate with
the rotational variations, therefore the other set of windings
produces synchronous alternating current (50/60Hz)
The initial part of the curve is
given by the power formula
Power
MW
4.5
4.0
3.5
3.0
3
1
P = _ A ρv WTGefciency
2
The system is feed from the output AC,
which is then converted to DC, and then
converted back to AC in the frequency
and phase needed by the generator
The WTG constrains and capabilities limit and
stabilize the power output into its rated power
Rated power
This curve represent how blades
would stall and lose efciency
if it wasn't for pitch control
2.5
2.0
1.5
1.0
0.5
4
2
Cut in speed
The minimum wind
speed for operation
6
8
10
Rated speed
16
14
12
wind speed m/s
The minimum speed needed
to achieve rated power
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18
20
26
22
24
Re-cut in speed Cut out speed
The average speed needed
to restart operation
The maximum speed
for safety operation
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Structure
New materials have made possible for turbines to keep growing, but the structural design of WTG is a quite a
complex subject, as modern turbines are massive moving objects subjected to changing forces due to the variability of
forces, turbines must withstand the constant changes without losing efciency or having major wear on its components.
The Thrust force on the blades
will cause a momentum on the
rotor, most of it will cancel
out by the momentum
of the other blades
The sum of all the components weight will be added
to the momentum forces created by the Thrust,
vibrations, and air drag over the entire turbine
Any variation on the position,
wind speed, misalignment
or unbalance on the blades
will create additional
momentum forces that will be
transmitted through the rotor
and create vibrations
The Thrust and drag will create a
sheer force through the tower
Wind shear is an important issue as it can cause
a mismatch on the forces, as the top blade will
have more thrust than the lower ones
The nal result of all the forces at the bottom
of the tower will vary since the air speed, direction
and blade positions are constantly changing
All the added forces will be transmitted through the
tower to the foundation which will dissipate them
into the ground, the soil need to have the
necessary strength to manage this forces
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Wind Farm development & construction
Most of this section is a working progress, keep checking
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Designed and created by César Sierra
csierra@simplerenewables.com
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The Wind Measurement campaign
The wind measurement is an essential although lengthy process that any large scale wind energy project must
undertake, a successful campaign will result with the selection of an ideal site and the optimal turbine for that
specic site, in addition a successful campaign will result in a precise production forecast that in the end will provide
a nancially healthy project.
Designed and created by César Sierra
csierra@simplerenewables.com
First Published in 14/10/2020
Version 1.3
Last updated in 12/04/2021
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Site Selection for wind projects
What is the rst step in developing a wind project?
In most cases it will be the collection of multiple information sources related with the needs of the project, a demand
for energy, a way to transmit power, available land and most important windy areas (resource) a Geographical
Information System is the perfect way to combine and visualize multiple layers of information in order to highlight
essential information. High resolution wind maps are a combination of metadata obtained of multiple sources and
processed to show the fundamental information.
Transmission map
+
Roads, urban areas &
infrastructure projects
Transmission voltage
Projected grid works
Consumption areas
Line saturation
+
Constrains map
Geographical limitations
Protected areas
Possible natural
disasters
Resource map
Satellite resource data
Nearby site measurements
Usual error is between
2.0 and 0.5 m/s
Archaeological sites
GIS can integrate multiple and
complex spatial data with the goal
of highlighting areas of interest.
Land availability
Environmental impact
Social Impact
(Geographic information system)
GIS Map
Site access
Potential consumers
Once all the layers are added a regional,
detailed, high resolution map is extracted,
showing the main areas of interest
Existing wind farms
Available infrastructure
Areas of interest where site scouting will
be done, are marked on the GIS map
Existing MET stations
High resource area
Wind farms
Intermediate resource area
Protected areas
Urban areas
Low resource area
Local constrains
Sub-stations
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Conventional
power plants
Power lines
Power consumers
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The MET station
The Meteorological station is the backbone of a wind development project and essential for lowering the risk taken
to develop and nance a wind project.
It's important to know Where to install the station? and What to install on them? For the rst question there is a series
of methods to determine the ideal spot, regional wind maps & topographical data can be simulated to nd ideal
spots, and satellite images help identify local constrains, however a site visit is always useful as there are always
accessibility and geographical issues that affect. For the second question I say the more the merrier, adding a few
more sensors will increase the station precision and reliability while only increasing a fraction of the overall cost of
the station. Met stations are still key elements in the development and operation of wind farms but new technologies
are looking to improve their performance or maybe even replace them.
Redundancy is key, multiple sensors
help correlate the data or as
back up for malfunctions
Current met towers can't keep up with the
increasing heights of WTG, multiple
measurement methods help achieve a
complete assessment
Having different sensor
models increase redundancy
The anemometer measures horizontal wind speed,
there are multiple classes that determine precision
and reliability
120m
Hub height
Wind vanes measure horizontal wind direction,
its critical to align and orient them correctly
100m
Maximum cord
Temperature, pressure, and humidity will directly
affect the density of the air and the amount of
energy produced
80m
The vertical wind speed helps correct
measurement errors, wind true angle of
attack & turbulence
60m
Tip of the blade
Multiple height measurements help calculate
the wind gradients or wind shear
The sensor layout is
usually designed for
a specic WTG
011001
7.58
The datalogger decodes and records all the sensor
signals it also stores and communicates the data
A power unit and back up batteries
keep the station running 24/7
Stations should be located where the
measurements are the most representative
Stations have a sphere of inuence that
can be limited by geographical features
A preliminary layout helps
select the correct spots
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Site accessibility and site constrains
inuence the selection of the site
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SODAR’ & LIDAR’
What will we do now that new turbine heights are unreachable for met masts?
The solution may lie on new technologies such as SODAR's and LIDAR's that are now allowing us to complement the
measurement campaigns, study the behavior of existing WTG's and even reach out into new sites that previously
where to difcult or expensive to access. SODAR and LIDAR technologies are becoming increasingly widespread
mainly due to the multiple applications and life span of a single unit, adding to this, is the cost reduction that these
systems have had over the last years.
Outgoing wind
separates waves
Stationary air bounces
the same wavelength
Incoming wind pushes
waves together
Doppler Effect
The LIDAR works by
beaming a laser array
and detecting differences
in laser return times
and wavelengths
Both systems are easy to
transport and require a
remote energy source
+200m
LIDAR
(Light detection and
ranging)
The SODAR bounces high pitch sound on the
wind layers, and detects the differences on
wavelengths that bounce back due to air
pressure differences
SODAR
Met mast height extrapolation
& calibration
SODAR's & LIDAR's
(Sonic detection
and ranging)
Wind speed 3D direction & turbulence
The result is a wind prole at multiple altitudes
Power curve
measurements
Performance optimization
Nacelle mounted LIDAR
Crane safety monitoring
Turbine alignment
Supports MET mast by lling
in the gaps on a site survey
Can be airlifted
to complex sites
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New site exploration
Offshore measurements for
monitoring or prospecting
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MET Data anlysis
On average a MET station generates 6 packages of information every hour, each package contains the 10min
average reading of about 10 to 15 sensors, when we start to add up the amount of information that a group of
stations generate on a year it can become to much to handle. Fortunately automatization and new data management
technologies enable us to be manage “big data”, and new algorithms also show us analyzed data and reports
sometimes in real time, now a days most resource analysis charts come from automatized reports, however it is
always convenient to know how to process all this raw data by yourself.
Each sensor emits a specic signal
that the logger interpretates the
RAW data
10min averages are widely used, shorter
averages increase information but require
better data handling
011001
7.58
Each met station has an average
data acquisition, ideally over
95% if the acquisition is poor,
maintenance must be done
Loggers compress, encrypt and send the
information in bundles that contain the
average measurements
If a station stops communicating
a physical download of the data
can be done by a technician
A server receives,
organizes and store all
the unprocessed data
The main outcomes of the analysis
Erroneous readings or
abnormal weather
Wind Rose, with wind
speed and frequency
Vav
Average Wind
speed
Specialized softwares can
manage multiple stations,
process the data and
report problems
m/s
Daily wind prole
m/s
Annual wind speed prole
m/s
Wind speed distribution
Vav
Vav
Vav
Important factors when selecting a WTG
Maximum wind gusts & turbulence
Wind classes for WTG
IEC 61400
Operating temperature range
Sustained gust registered
In the last 50 years
Maximum gust registered
in the last 50Yrs
Average wind speed
Turbulence is the amount of times wind
changes speed or direction in 10min
IA
m/ Vav
s
I 10
II 8.5
III 7.5
A
IIB
IIIB
Vgust
Vref
70
60
53
50
43
38
B
C
+16% 14% 12%
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High humidity and low freezing
temperatures cause icing on blades
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Layout design
This section is a working progress, keep checking
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Designed and created by César Sierra
csierra@simplerenewables.com